As is well known, recently, the development of processor manufacturing technique, increment of memory capacity and development of multimedia encoding tools allow a variety of additional functions to include within portable devices such as cellular phones, personal digital assistants (PDAs) and so on. Moreover, the menus for those additional functions in the portable devices are practically being enlarged, and thus, key pads are not good enough anymore to represent those various functions in the portable devices.
Further, in view of a trend in which a number-based or text-based user interface is transited into a graphic-based user interface, it is difficult to utilize efficiently graphic-based application programs running on the portable devices with ordinary number, letter and direction keys. In addition, a demand for a display device with a wide screen is increasing as the portable devices include a variety of image display functions for a multimedia reproduction, a communication function for a mobile internet or others.
Moreover, in recent, the portable devices employ a touch screen in which functions to perform key and display operations are implemented in one unit without separate key pad. The touch screen allows users to use the application programs interactively and perceptively, by touching a button or a graphic icon on a display area with a finger or a pen, which results in a simplified manipulation. Also, the graphic icons for the key operation can be easily identified by users as they are optimized for the respective application programs. Therefore, the key operation becomes easier.
In addition, since a touch pad is integrated with a display device in a single body, the touch screen does not require an additional space for key pad, unlike a conventional portable device. Therefore, the touch pad may have an advantage that it can be adapted to the portable devices to provide a wider display screen.
The touch screen is classified into a capacitive overlay system, an infrared beam system, a surface acoustic wave system, a Piezo electric system, an integral strain gauge system, and a resistive overlay system. For example, the capacitive overlay touch screen uses sensing data represented on each of “x” and “y” axes to control various operations on the touch screen.
Meanwhile, FIGS. 1A and 1B show capacitive touch pads of a related art. FIGS. 2A and 2B are graphs illustrating data detected when a capacitive touch pad of a related art is touched by an object. The capacitive touch pad determines whether or not an object is touched by the following Equation.τ=R×(Cp+Cf)  [Equation]
In Equation, “τ” represents a time constant in a circuit with a resistor and a capacitor, “R” represents a resistance of the touch pad, “Cp” represents a capacitance of the touch pad, and the “Cf” represents a capacitance caused by a touch of the object to the touch pad. If the object is approaching the touch pad, the capacitance Cf is varied and in turn the time constant “τ” is varied. Accordingly, it is possible to determine whether or not the touch pad is touched by continuously monitoring the time constant.
The capacitive touch pad of a related art may have a diamond pattern shown in FIG. 1A or a layer bar pattern shown in FIG. 1B. If the capacitive touch pad is touched by an object, it is possible to get data on x-axis shown in FIG. 2A and data on y-axis shown in FIG. 2B. In this case, a coordinate of a touched position is (4, 7) in the capacitive touch pad. Therefore, an operation corresponding to the coordinate may be performed in a touch screen.
FIGS. 3A and 3B shows graphs illustrating data measured when a capacitive touch pad of a related art is touched at multiple points by objects. For example, if the capacitive touch pad is touched at multiple points by two objects, it is possible to get data on x-axis and y-axis as shown in FIGS. 3A and 3B. As known in FIGS. 3A and 3B, the data are 1 and 5 for x-axis, 2 and 8 for y-axis. In other words, the two touch positions have 4 coordinates such as (1, 2), (1, 8), (5, 2) and (2, 8). Consequently, the two touch positions have two real coordinate values and two virtual coordinate values.
As described above, when the capacitive touch pad is touched at only one point, it is possible to drive a touch screen because the touched position is easily recognized. However, if the capacitive touch pad is touched at multiple points, virtual coordinates with real coordinates coexist, which entails a difficulty to identify the real coordinates.